Well, with the Alien movies the change in pacing was mostly because of changes in directors. Each director has their own style.
Ridley Scott (Gladiator, Blade Runner, Black Hawk Down) only directed the first movie.
Second movie is James Cameron (Titanic, Avatar, Terminator, T2: Judgement Day) who tends to go for large scale action movies. Big budgets, grandiose, he gravitates towards all of that.
Third Alien movie brings the horror back, I think. David Fincher (Fight Club, Se7en, Gone Girl) is a master of suspense and it shows. For me, Alien 3 was the most suspenseful of them all. They didn't even have guns to defend themselves and the new film-making technology of the time was not wasted. Also, the way they came up with to kill an alien without weapons was pretty brilliant.
Though it is true that, in general, horror movie sequels have stepped towards action. I don't think it's so much that action has more appeal that horror is harder to sustain. It's really hard to keep something scary when you used your bag of tricks once already. It's more difficult to make a movie the audience won't see coming. Like, Friday the 13th 11 comes out, is Jason Voorhees really going to surprise anyone or are they there to see more of what they saw already?
The sequel has to add something and its easier to make that something a grander scale than a fresh and clever one.